Field of the Invention
The present invention relates to a coding format converter, and more particularly to a coding format converter for transcoding video streams of different coding formats.
Description of the Background Art
A compression coding of video information based on a video coding format, such as MPEG-2 and H.264/MPEG-4 AVC (Advanced Video Coding) formats, which are standardized by The Moving Picture Experts Group (MPEG), is implemented in such a way that input signals of subject moving pictures are divided into blocks as processing units, for each of which a prediction residual signal, i.e. a difference between a prediction picture with motion predicted and compensated for and an input subject picture, is spatially transformed by, e.g. discrete cosine transform, to thereby obtain transform coefficients, which are in turn quantized and subsequently entropy-encoded, thus attaining high-efficient video compression.
In recent years, a new ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) and ITU-T (International Telecommunication Union Telecommunication Standardization Sector) standard, H.265/MPEG-H HEVC (High Efficiency Video Coding) , has been defined to accomplish a better image quality and higher compression efficiency.
According to HEVC, the size of blocks of 16×16 pixels, used as macro-blocks in a conventional video coding format such as AVC, is expanded so as to be able to represent the structure of coding units (CUs), having a block size variable from 8×8 to 64×64 pixels, inclusive, with a quad-tree structure to thereby allow the coding with a variety of block sizes, see FIGS. 2A and 2B.
Moreover, the CU can further be divided in its vertical and/or horizontal directions into several prediction units (PU). The motion compensation prediction and other processing can be performed on subunit regions formed by dividing the CU of 2 N×2 N-pixel block size into the size of 2 N×2 N, 2 N×N or N×N, or an asymmetrical shape, where N is a natural number, see FIGS. 3A, 3B and 3C.
Currently, a variety of video coding formats including HEVC has been used, and thus there have been demands for transcoding techniques applicable to transcoding video streams in different coding formats.
In general, when a video stream is converted from one coding format to another, the video stream in a first coding format, i.e. an input stream, is decoded into data of a decoded picture, which is then encoded into a second coding format.
For example, U.S. Pat. No. 7,526,027 to Sekiguchi et al., discloses a method for performing such transcoding efficiently, in which motion vector information derived from a stream in a first coding format is used to carry out encoding based on a second coding format.
HEVC can implement various combinations of CUs with PUs, unlike conventional coding formats. Therefore, when using HEVC as a second coding format, selection of the optimum one of the various combinations significantly influences the coding efficiency. In order to select the optimum combination, the coding cost, closely associated with the coding efficiency, has to be estimated on a considerable number of combinations of coding format candidates, and it is therefore important how efficiently to search for coding candidates.
By way of example, K. Choi et al., “Coding Tree Pruning Based CU Early Termination”, JCTVC-F092, July 2011, (http://phenix.it-Sudparis.eu/jct/doc_end_user//current_do cument.php?id=2555) teaches a conventional example of solution for dividing CUs, in which the coding costs are evaluated for a given CU size in an inter-prediction mode/intra-prediction mode, when the result of which shows that a skip mode is considered optimum, no search will be conducted for further dividing the CU into a smaller size of blocks.
R. H. Gweon et al., “Early Termination of CU Encoding to Reduce HEVC Complexity”, JCTVC-F045, July 2011, (http://phenix.it-Sudparis.eu/jct/doc_end_user//current_do cument.php?id=2508) teaches a conventional example of solution for dividing PUs, in which the coding is evaluated through the inter-prediction for a given candidate for division into PUs, and when the result of the evaluation shows no prediction residual, which means not proceeding to encoding prediction residual information, no search will be conducted for further candidates for division into PUs.
J. Yang et al., “Early Skip Detection for HEVC”, JCTVC-G543, November 2011, (http://phenix.it-Sudparis.eu/jct/doc_end_user//current_do cument.php?id=3800) teaches another conventional example of solution for selecting CU modes, in which coding candidates are evaluated through the inter-prediction for the 2 N×2 N size of PUs, and if the result of the evaluation shows that there are neither motion vector difference nor prediction result information, the search will not be carried out for either of further PU division candidates for the inter-prediction and coding candidates based on the intra-prediction candidates.
Those conventional solutions for reducing the candidates for searching for dividing CUs and PUs are, on one hand, effective in reducing the workload required for the coding processing, and, on the other hand, cause some problems, such as decrease in coding efficiency and deterioration in image quality.